Ultrafast dynamics in the metal-to-ligand charge transfer excited-state evolution of [Ru(4,4 '-diphenyl-2,2 '-bipyridine)(3)](2+)

The transition metal complexes [Ru(dmb)(3)](2+) and [Ru(dpb)(3)](2+), where dmb is 4,4'-dimethyl-2,2'-bipyridine and dpb is 4,4'-diphenyl-2,2'-bipyrid ine, have been studied by femtosecond visible electronic absorption spectro scopy. Spectroelectrochemical measurements in conjunction with nanosecond t ime-resolved absorption spectroscopy allow for the assignment of various fe atures in the excited-state differential absorption spectra as both ligand- based pi* <-- pi* and ligand-to-metal charge transfer (LMCT) in nature. A u nique absorptive feature centered at similar to 530 nm in [Ru(dpb)(3)](2+) was identified as an optical marker for the thermalized (and hence fully in traligand delocalized) excited state. Single wavelength and full spectrum t ransient absorption data were obtained on both molecules in CH3CN solution at room temperature following metal-to-ligand charge transfer (MLCT) excita tion at 400 nm. Data on [Ru(dmb)(3)](2+) at 532 nm, a region of net excited -state absorption, revealed biphasic decay kinetics (similar to 120 fs and 5 ps) attributed to a combination of (MLCT)-M-1 --> (MLCT)-M-3 intersystem crossing and vibrational cooling dynamics. Dynamics for [Ru(dpb)(3)](2+) un der identical conditions revealed biphasic rise times in the region of the ligand-based pi* <-- pi* absorption at lambda(probe) = 532 nm. Although the origin of the fast component (similar to 200 fs) is not yet clear, the ca. 2 ps rise is assigned to rotation of the peripheral aryl ring and thus cor responds to the time scale for intraligand electron delocalization.